Its simply two colors of Buttercream, Spread together, its been done very well in that cake so as to not look odd, Marbled buttercream if you will
Seriouscakes has a video
I don't think it is a marbled icing in my opinion. But it could be. Personally I think you could get the same effect by doing it all white and then using petal dust with a puffy brush. Maybe do a trial run and try marbeling and try the petal dust or whatever other techniques others list. You don't even have to put the icing on a cake, I sometimes do trials by just smearing it thinly on some parchment paper. Good luck.
Agree with Sunset74, this looks like petal dust dusted onto crusted buttercream - very effective.
I agree with sunset 74, it looks like they have dusted blue petal dust on it.
I think you should just email them and ask them. Everyone is really helpful on here.
I have some extra icing, I'll play around with it on my day off tomorrow. Thanks for the suggestions : )